Additional feature to fetch api-docs from file path

1) Raise an error if swagger is fetched from file path and basePath of
API is '/', as there is no host/port to call as otherwise was the case
with fetching spec from http. It used the same host:port in that case.

2) Add a third optional parameter 'app_base_path' to SwaggerClient
to cater point no. 1

3) There are two ways to load spec from file path:
  3.1) It needs spec files to be present at one level (desired):

  from swaggerpy.swagger_model import load_file;
  client = SwaggerClient(load_file('path/to/api-docs[.json]'),
app_base_path='http://foo')

  3.2) It needs nested level. resources to be present under api-docs/

  client = SwaggerClient('file://path/to/api-docs', # NO .json needed
app_base_path='http://foo')